It is known from US 2007275676 that modern wireless RF transmitters for applications such as cellular, personal, and satellite communications, employ digital modulation schemes, such as frequency shift keying (FSK), phase shift keying (PSK), and variants thereof, often in combination with code division multiple access (CDMA) communication. The RF transmitter output signal will have an envelope which in some of the above mentioned communication schemes is a constant envelope, and in other communications schemes will vary with time. An example of a variable-envelope modulation scheme is that known as a polar transmitter. In a polar transmitter, digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to some intermediate frequency (IF) carrier fIF to generate digital envelope (amplitude-modulated) and digital phase-modulated signals. The digital amplitude-modulated signal is input to a digital-to-analog converter (DAC), followed by a low pass filter (LPF), along an amplitude path. The digital phase-modulated signal is input to another DAC, followed by another LPF, along a phase path. The output of the LPF on the amplitude path is an analog amplitude signal, while the output of the LPF on the phase path is an analog reference signal. The analog reference signal is input to a phase locked loop to enable the phase of the RF output signal to track the phase of the analog reference signal. The RF output signal is modulated in a non-linear power amplifier (PA) by the analog amplitude-modulated signal.
Thus, in polar transmitter architectures, the phase component of the RF signal is amplified through the non-linear PA while the amplitude modulation is performed at the output of the PA. This architecture, however, requires phase and amplitude alignment to make sure that the amplitude modulated and phase modulated data are applied at the right instant. In addition, polar transmitters also have several challenges related to amplitude modulation and power control. Conventional amplitude modulation techniques are typically based on the modulation of the power supply. However, the amplitude component of the RF signal occupies several times more bandwidth than the combination of the phase and amplitude data. Therefore, conventional power supply modulation techniques are limited for many wideband applications. In addition, in many wireless systems, the output power must be controlled in order to keep the received signal from reaching all users at the same power level. However, in switching power amplifiers, the power control is performed using the same method as that used for amplitude modulation. As a result, in switching power amplifiers, there is a trade off between the power control dynamic range and the resolution of the amplitude modulation. Furthermore, the AM signal path needs to be extremely linear. Any distortion leads to unacceptable spectral power emissions (“spectral leakage” or “spectral regrowth”) in neighbouring transmit channels.
A multi stage power amplifier is provided in the above mentioned US 2007275676 to address problems with Local Oscillator leakage. The leakage current is orthogonal to the drain current of the switching transistor due to the 90° phase difference of the capacitor's voltage and current. As a result, when the amplitude modulation is applied, there is a variation in the carrier's phase due to the leakage which is a function of the carrier's envelope (amplitude). This effect is known as the AM to PM conversion, and is critical when the power amplifier operates at high output power level. To compensate for the AM to PM conversion in polar transmitters, a pre-distortion filter or phase feedback loop can be employed. In addition to or in the alternative to using a pre-distortion filter and/or phase feedback loop, cascode transistors can be used on top of the switching transistors, which also reduces the voltage variation over the switching transistors, and therefore reduces the AM to PM conversion.
In low power operation, the LO leakage through the CGD capacitor can be comparable or even higher than the output RF signal. As a result, the leakage covers the RF signal at the output, and therefore limits the power control dynamic range. The leakage signal may also limit the linearity of the amplitude modulation at low power operations. To overcome the leakage problem at low power, the switch size is decreased for low power levels by providing multiple stages in the power amplifier. For example it can be divided to three stages with switch size ratios of *1, *8 and *64. Each stage includes a switch pair and a corresponding tail current source. Each current source is operably coupled to receive the amplitude-modulated signal and the power control bits to control current through its respective switch pair and each stage is associated with a different power level to minimize leakage at low power levels. Switches connected to each switch pair select one or more of the stages to generate the appropriate output power for the power amplifier, or the stage selection can be made using the most significant bits (MSBs) of the power control word to turn on the appropriate stages. The remaining least significant bits (LSBs) can be used to control the tail current. Turning off the large stages improves the linearity of the power amplifier in low power operation.
It is also known to use a so called “RF DAC” or “Envelope DAC”, which is essentially a switching RF power amplifier combined with a multi-bit Nyquist DAC. See e.g. P. T. M van Zeijl, M. Collados, “A Digital Envelope Modulator for a WLAN OFDM Polar Transmitter in 90 nm CMOS”, IEEE Journal of Solid-State Circuits, October 2007. The “RF DAC” (=the PA) can provide predictability/exactness in time, necessary for correct AM and PM recombination, by directly feeding the digitised amplitude data into the PA. Hence the propagation delay can be known to within 100 ps. The amplitude exactness however depends on the device matching of the binary weighted unit cells that constitute the PA. As these devices need to operate at RF, usually several GHz, their dimensions need to be small. Consequently the matching will be poor (matching scales with area). To alleviate this problem the “RF DAC” can be constructed from thermometer encoded unit cells, rather than binary weighted unit cells. The impairments can be engineered to an acceptable low level, but in principle, device mismatch will impair faithful reconstruction of the AM signal.